Water-cooled nuclear reactors, particularly pressurized-water nuclear reactors, have assemblies consisting of a bundle of fuel rods of great length which are arranged parallel to one another and held in a framework formed by guide tubes, spacers and two end connectors. The guide tubes are arranged in the longitudinal direction of the assembly and are connected to transverse spacers uniformly spaced over the length of the assembly.
The guide tubes are also connected, at each of their ends, to a connector forming an element for stiffening and closing the assembly.
The fuel rods of the assembly form a bundle, in which the rods are parallel to one another and, in the cross-sections of the assembly, arranged according to a regular network determined by the spacers. Some positions of the network are occupied by guide tube which are usually connected rigidly to the spacers.
The guide tubes are longer than the fuel rods and are arranged in the bundle so as to have a part projecting relatively to the bundle of fuel rods at each of their ends. The connectors are fastened to these projecting parts of the guide tubes, in order to ensure that the assembly is closed at each of its ends.
The fuel rods consist of sintered pellets of nuclear fuel material which are stacked inside a metal sheath isolating the pellets from the fluid surrounding the fuel assembly. If the sheath of a rod of a fuel assembly breaks, this rod has to be replaced very quickly in order to prevent radioactive product from leaking into the cooling fluid of the reactor. To gain access to the fuel rods and carry out their replacement, it is necessary to remove one of the connectors of the assembly, and this requires breaking the connections between the corresponding ends of the guide tubes and the connector.
The connectors have passage holes which reproduce the network of guide tubes and in each of which a guide tube is engaged and fastened.
In the most customary technique, the guide tubes are fastened permanently to the connectors, usually by welding, and it is not possible to remove the connector in order to carry out the replacement of a rod. It is therefore necessary, should the sheath of a rod break, to replace the defective fuel assembly by a new fuel assembly. The defective fuel assembly is dismantled, and the non-defective but highly irradiated fuel rods contained in this assembly are recovered and reinstalled in a new fuel-assembly framework. To carry out these operations and manipulations of contaminated elements, it is necessary to take precautions and work in complex and costly installations. The recovery of the non-spent fuel rods in the defective assemblies is therefore an operation involving a very high outlay.
New fuel assemblies having guide tubes, of which the connection to at least one of the end connectors is removable, have consequently been designed and developed.
The assembly is placed underwater in the vertical position, in a pool, such as a fuel-assembly storage pool, in which the replacement of the defective fuel rods is carried out. The guide tubes of the assembly are in vertical position, the assembly resting on the bottom of the pool by means of the lower connector. The upper connector is accessible, under a certain depth of water, from the top of the pool. Those parts of the guide tubes engaged in the upper connector of the assembly have a radially expandable part which, for example, can be attached to the end of the guide tube. This radially expandable part consists of a split bush having a radially projecting part which is intended to come to rest in a cavity of corresponding form machined inside the connector, in the passage hole of the guide tube. A blocking sleeve introduced into the guide tube ensures the radial expansion of the split bush and the retention of the guide tube.
When the removal of the upper connector and then the extraction and/or replacement of some rods of the assembly are carried out underwater in a pool, the upper connector has to be put back in place and fastened to the guide tubes of the assembly. For this purpose, it is necessary for a blocking sleeve to be introduced into the upper part of each of the guide tubes and fastened.
Devices used both for the fitting and for the extraction of the blocking sleeves are known. Such devices comprise a split tubular bush which terminates in a radially projecting bead and in which a stem terminating in a knob can be moved in order to actuate the split bush so as to move apart the sectors of the split bush. Such a device is not entirely reliable and there are some difficulties in using it.
No tool both simple and effective, making it possible to carry out the fitting of the blocking sleeves in the guide tubes of the removable assembly, has been known to date.